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I'm designing a board with an RF L-band trace (around 2.150GHz) on the top layer side. I have a clear and unbroken ground plane on the bottom layer of the trace. However, I'm curious to know whether do I need to apply a ground polygon on the top layer side of the PCB or not? I've seen that there is no adjacent ground polygon on several samples besides the RF trace. Does it have any specific reason? Thanks,

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2 Answers 2

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do I need to apply a ground polygon on the top layer side of the PCB or not?

To make a microstrip transmission line, you do not.

And if you do, you should keep the top layer ground region well away from the trace. At least 3x the trace width, preferably 5x or more.

Does it have any specific reason?

Because microstrip is designed with only a trace on the top layer and a ground plane on a lower layer.

If you put ground planes on the top layer, then you increase the capacitance from the trace to ground, and change the characteristic impedance of the trace. The design equations and the computerized calculators that you might use to design your microstrip won't account for this effect.

You can account for that if you design your waveguide as (conductor-backed) coplanar waveguide, but then you have coplanar waveguide, not microstrip.

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Whatever you do, make sure you pay attention to the geometry of the stackup. If you put a plane above and a plane below, the transmission line changes from a microstrip to a stripline. Notice the dimensions of the traces and the H1 and H2 dimensions. If you do create a transmission line you'll need to make sure you use a calculator to maintain the impedance of the line if you switch geometries.

enter image description here Source: https://madpcb.com/glossary/transmission-line/

Make sure you keep the reference plane (or planes) continuous. This is important at 2GHz, because the return current will flow directly adjacent to the transmission line trace and any inductance can slow the return current down giving you worse eye diagrams or signals.

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